It is known that, in order to obviate the fragility of thermoplastic materials, a generally used technique consists in blending them with relatively small amounts of elastomers.
According to these techniques, certain amounts of elastomers, typically within the range of from 5 to 35% by weight, are incorporated and dispersed inside the rigid thermoplastic matrix and the resulting material is thereby given characteristics of impact and crack resistance, elongation at break, and crack energy several times higher than the thermoplastic resin used as the starting material.
In case of transparent polymers such as, e.g., the acrylic resins, dispersing inside them an elastomeric phase according to the known techniques may have an adverse effect on optical properties, such as an increase in haze, which is the more marked the more different from each other are the refractive indices of the continuous resin phase and of the dispersed elastomeric phase.
In order to obviate such a drawback, compositions were proposed, e.g., in U.K. patents Nos. 1,001,953 and 994,924, which are based on acrylic resins, such as polymethylmethacrylate, intimately blended with grafted materials comprising an elastomeric core coated with analogous acrylic resins.
Finished articles obtained from these compositions showed good mechanical properties, and values of optical properties similar to those of the acrylic resin used as the starting material, at room temperature, but they tended to opacify at even slightly different temperatures.
Further transparent compositions were subsequently proposed, wherein the above drawback was overcome.
These compositions, comprising a continuous phase, the acrylic resin, and a dispersed phase constituted by substantially spherical polymeric particles of multi-layer type, make it possible to obtain finished articles endowed with satisfactory optical properties over a wide temperature range.
The multi-layer particles consist of a central core, based on acrylic resins, on which are grafted a plurality of layers of resin, or of acrylic elastomer, having different compositions.
But in this case too, the compositions are not without drawbacks; in fact, such multi-layer products, obtained in aqueous emulsion, are constituted by particles having variable dimensions and diameters, with values which, although having an average value within the required limits to provide a reinforced thermoplastic matrix, are distributed over very wide ranges.
This fact may gave an adverse effect on the properties of the finished articles, in that the particles having dimensions smaller than the average value turn out not to be efficacious in improving the mechanical properties, and the particles having larger dimensions can reduce the optical qualities of the end products to which impact-resistance properties have been imparted.
Besides the above, the end articles obtained from acrylic resins modified with the multi-layer polymeric products, even if they show rather satisfactory optical properties with varying temperatures, nevertheless show a more or less light blue hue, which cannot be eliminated even if suitable optical modifiers are used. Such a phenomenon, generally indicated by the term "color reversal", can be observed in transparent heterophase systems when the refractive index of the dispersed phase is even only slightly higher than that of the continuous phase.
Thus, such end articles have poorer aesthetic properties than those of the acrylic resin as such.